Development of measures to meet the improvement of paper quality and the employment of the closed water recycling system is an issue of urgent necessity in the papermaking industry. It is also confronted with difficulties involved in using waste paper and brokes containing calcium carbonate as materials for the production of paper. In order to cope with these problems, papermaking in the neutral range with reduced amount of alum (crude aluminum sulfate) has been more commonly conducted.
Fortired rosin sizing agents are usually used in conventional papermaking systems wherein significant amount of alum is added. The sizing effect of these sizing agents, especially of rosin soap sizing agents comprising an alkali salt of a fortified rosin, falls sharply as the addition amount of alum is decreased and the pH of the papermaking system is made higher up to near the neutral papermaking range. The deterioration of the sizing effect is conspicuous when the papermaking system contains calcium carbonate. To prevent the deterioration of the sizing effect, addition of alum in a large amount is necessitated after all, which lowers the pH of the papermaking system to the acidic range, which in turn causes degradation of the quality of paper and other operational and economical problems.
Cationic rosin emulsion sizing agents, which can be obtained by dispersing a fortified rosin in water as finely divided particles with the aid of a cationic emulsifying and dispersing agent, have properties of fixing onto the pulp fiber by themselves and require less amount of alum to develop sufficient sizing. It is also known that they exhibit an excellent sizing effect in papermaking systems of almost neutral pH ranges (See TAPPI Papermakers Conference 1988, pp 181-187).
However, improvements are still required of such cationic rosin emulsions. That is, their preparation on a commercial scale is relatively difficult compared with that of conventional anionic rosin sizing agents and their storage stability, sizing performance and foaming properties in a papermaking system containing calcium carbonate are not satisfactory.
Under the circumstances, neutral papermaking sizing agents for use in papermaking in the neutral range have been developed, examples of which include sizing agents prepared by dispersing alkenyl succinic anhydride, alkyl ketene dimer and the like in a dispersion medium such as water. The major problem associated with these agents is that they incur increase in cost for the production of paper because either one of the above-mentioned compounds is expensive. They also present operational problems, being apt to cause contamination in the papermaking system, for instance, on the press roll and the like. Furthermore, they are slow to develop their sizing effect.
Rosin sizing agents having an improved sizing effect in neutral papermaking systems are proposed in Japanese Laid-open Patent Publication Nos. 62-223393 and 62-250297 which disclose sizing agents comprising an ester compound of a tri- or quadrihydric alcohol and a rosin compound. Some of the drawbacks associated with these sizing agents are that they require ester compounds in such an amount that their content is over 20 wt % preferably 40 wt %, that their sizing effect in acidic papermaking systems is not satisfactory and that they incur increase in production cost since the esterification reaction between such an alcohols and a rosin proceeds slowly and requires prolonged heating in order to attain a sufficiently low acid value.
As described above, there is a demand for a rosin emulsion sizing agent which rapidly develops an excellent sizing effect in papermaking systems wherein the addition of alum is limited to a low level and the pH is increased up to around neutrality and which shows excellent mechanical and storage stabilities, and remarkably reduced foaming in the papermaking system, especially such a sizing agent for neutral papermaking systems containing calcium carbonate, and sized paper and a sizing method using the same.